1. Field of the Invention
The present invention relates in general to a method of friction stir welding, and more particularly to a method of friction stir welding suitably applicable to an operation of joining together two end sections of a generally cylindrical body which is formed from an aluminum plate such that the two end sections are butted together along a joint region or line. The generally cylindrical body the end sections of which have been jointed together by friction stir welding is subsequently subjected to a forming operation such as a roll forming process, to provide a desired tubular end product such as a wheel for automobiles, or a tubular product which is required to exhibit a sufficiently high strength or resistance at its weld region to internal pressure.
2. Discussion of Related Art
In the art of manufacturing a tubular product which has a relatively large diameter and a relatively small cylindrical wall thickness and which is considered difficult to be formed by extrusion as an integral body, there have been employed various methods wherein a starting workpiece in the form of a plate or sheet is formed into a tubular body by roll forming, and the end sections of the formed tubular body are butted and urged together along a joint line parallel to an axis of the tubular body, and are joined together by a suitable butt welding process such as MIG welding (metal inert-gas welding), TIG welding (tungsten inert-gas welding), flash butt welding, or DC butt welding. Such a butt welding process involves melting of the material, and is a sort of fusion welding process, which tends to suffer from occurrence of defects at the weld region under some welding conditions, giving rise to a risk of significant reduction in the yield ratio of the product.
Recently, an increasing attention has been drawn to a friction stir welding process which is a sort of solid-phase welding process and which involves a relatively small amount of heat generation in the workpiece, a relatively small amount of strength reduction in the weld region and a relatively small amount of residual strain in the weld region. The application of this friction stir welding has already been employed for butt welding in the manufacture of a tubular body for a rim of an automotive wheel. In the friction stir welding operation in general, two workpieces to be joined together are butted together along a joint line or region, and a rotary tool or wheel provided with a probe or pin extending from its shoulder surface (bottom end face) is rotated and moved or translated relative to the workpieces, while the shoulder surface is held in rubbing contact with the surface of the joint region, with the probe being rotated with the rotary tool and inserted in the joint region, so that frictional heat is generated to plasticise the material at the joint region while causing plasticised segments of the joint region to be interlocked with each other, as a result of a stirring action of the probe rotating at a high speed, whereby the two workpieces are joined together without melting of the material.
Where the friction stir butt welding is practiced on a joint region defined by the butted adjacent surfaces of a tubular or cylindrical body, the shoulder surface of the probe or pin of the rotary tool is held in a rubbing contact with a part-cylindrical surface at and near the joint region, such that the depth of undercut of the shoulder surface into the material is relatively large at a radially central portion of the shoulder surface near the probe inserted in the joint region, and is relatively small at a radially outer portion of the shoulder surface. Accordingly, the plasticised material is likely to be expelled or driven from a local area of the joint region corresponding to the radially central portion of the shoulder surface of the rotary tool, toward a local area corresponding to the radially outer portion of the shoulder surface, so that the weld region obtained along the joint line tends to be recessed at a central portion thereof corresponding to the radially central portion of the shoulder surface, so as to form an undesirable cavity extending in the direction of the relative movement between the rotary tool and the tubular body. The formation of the cavity in the weld region causes reduction in the mechanical strength of the welded tubular body. In this respect, the friction stir butting welding process has been considered inappropriate for butt welding in the manufacture of a generally tubular product such as a wheel for an automobile.
In view of the drawback experienced in the friction stir butt welding process described above, there have been proposed methods of manufacturing a two-piece aluminum alloy wheel for an automobile, as disclosed in JP-A-2001-88504. In the method disclosed in this publication, an aluminum alloy plate is formed into a generally cylindrical formed body, and the opposite end sections of the formed body are flattened before these flattened end sections are butted together along a joint line or region and then joined together by flash butt welding or friction stir butt welding. Subsequently, the flattened end sections jointed together along the joint line are re-shaped into a part-cylindrical shape, so that the thus obtained cylindrical body is used as a desired aluminum alloy wheel. JP-A-2000-142003 discloses a method of manufacturing an aluminum wheel for an automobile, wherein two workpieces in the form of two semi-cylindrical halves are machined at their end faces and are butted together at the machined end faces and then joined together into a truly cylindrical body of aluminum, by electron beam welding, laser welding, friction stir welding or any other welding process which involves a relatively small amount of heat generation.
While the publications identified above disclose the application of various welding techniques, a primary concern as described in those publications is the application of the electron beam welding and flash butt welding, and the application of the friction stir welding technique is merely referred to in the publications. Namely, the publications do not disclose the details on the conditions in which the friction stir welding process is performed, for example, the specific width dimension of each of the opposite flattened end sections of the generally cylindrical body, which width dimension permits the end product to have a high-quality weld region.
Where the friction stir welding operation is performed on a cylindrical body, it will be generally understood that the required width of the flattened end sections of the cylindrical body is sufficiently larger than the diameter of the rotary tool or wheel used. For increased efficiency of manufacture of the cylindrical end product, however, it is desirable to minimize the width dimension of the flattened end sections prior to the application of the friction stir welding, in view of a need of re-shaping the flattened end sections after their joining, into a truly cylindrical body as the end product. On the other hand, it will also be understood that the width dimension of the flattened end sections cannot be made to the diameter of the rotary tool, for all combinations of the specifications of the rotary tool and tubular body and the friction stir welding condition, in order to minimize the amount of generation of flash in a portion of the joint region corresponding to the leading outer edge portion of the shoulder surface of the rotary tool, irrespective of the diameter of the tubular body and the depth of undercut of the shoulder surface into the material of the joint region.